Particle accelerators are used today in various technological fields. As just one example, accelerated particles can be used to generate proton beams for irradiation of targets (e.g., enriched water or other materials) in order to produce medical isotopes. The resulting medical isotopes can be used as biomarkers, e.g., for medical imaging applications such as positron emission tomography (PET).
A collection of charged particles may be referred to as a particle beam. Various types of particle accelerators are used for accelerating particle beams. One type of particle accelerator is a linear accelerator. Another type of particle accelerator is a cyclotron, which is described at, e.g., U.S. Pat. No. 1,948,384 to Lawrence and U.S. Pat. No. 7,015,661 to Korenev, the entire contents of which patents are hereby incorporated by reference herein. A cyclotron accelerates a particle beam (including, e.g., ions such as negatively charged hydrogen ions) by using a rapidly varying electric field. Charged particles that are injected into a vacuum chamber are forced to travel along a spiral trajectory (e.g., with increasing radius for successive orbits) due to a magnetic field, which yields a Lorentz force perpendicular to the direction of motion of the particles. In an isochronous cyclotron, also known as an azimuthal varying field (AVF) cyclotron, the magnetic field strength varies dependent on azimuth of the particle beam along the spiral trajectory. For example, some azimuthal ranges correspond to magnetic hills and others correspond to magnetic valleys. The azimuthal variations in magnetic field strength balance the relativistic mass increase of the particle beam so that a constant frequency of revolution is achieved for the spiral motion.
An accelerated particle beam can be used for nuclear reactions for production of medical isotopes. Nuclear reactions associated with the irradiation of a proton beam upon a target material are often used for generation of medical isotopes such as C-11, N-13, O-15, F-18, Ge-68, Ga-67, Ga-68, Sr-82, Rb-82, Y-86, Tc-99m, I-111, I-123, I-124, Tl-201, or other isotopes. Photonuclear reactions (nuclear reactions resulting from the collision of a photon with an atomic nucleus) may also be used for production of medical isotopes. The production of medical isotopes through nuclear reactions based on target irradiation by a proton beam requires the production of such a proton beam. The standard approach for producing proton beams is to convert negative hydrogen ions into a proton beam and electrons using a stripper foil according to the following process:H−→p++2e−  (1)
Process (1) is referred to as a stripping process because electrons are stripped away from the protons. Process (1) may also be referred to as an electron-stripping or proton-stripping process.
Then, the nuclear reaction of protons with O-18 in enriched water yields the medical isotope F-18, for example. The yield of the isotope depends on various factors including beam current, beam kinetic energy, and time of irradiation. It is desirable to produce medical isotopes efficiently.
One approach for increasing the efficiency of isotope production is to adjust particle beam parameters to increase the beam current to yield an increased cross-sectional area for the stripping process, but increasing beam current causes thermal problems for the target. Another approach for increasing efficiency is to increase the number of targets and create multi-beam channels. A traditional implementation for irradiating multiple targets is shown in FIG. 1. Traditional stripper foils 130a and 130b are placed at different azimuths along an orbit of a spiral trajectory traversed by an accelerated particle beam. FIG. 1 shows a side view of the particle beam's trajectory, which proceeds from left to right in the figure. First, stripper foil 130a is encountered. As shown in the FIG. 1, about half the particles in the negative hydrogen ion beam 110 strike stripper foil 130a and are thereby converted to protons and electrons according to the process (1). The half of the particles in the negative hydrogen converted to protons and electrons are depicted as the upper half in the view of FIG. 1. As a result of the stripping process, each negative hydrogen ion loses two electrons in stripper foil 130a and is converted to a proton. The proton beam resulting from this stripping process is shown as 140a in FIG. 1, and the resulting electrons are not shown. The remaining particles in the negative hydrogen ion beam (denoted as 135 in FIG. 1) continue along their spiral trajectory because they did not collide with stripper foil 130a, and they subsequently collide with stripper foil 130b to yield proton beam 140b and electrons (not shown). Thus, two proton beams 140a, 140b are produced by respective negative hydrogen ion beams 110, 135 and can be used to irradiate respective targets.
The traditional multi-beam approach described regarding FIG. 1 presents several challenges. The position of stripper foil 130a (the foil encountered first along the trajectory) has to be carefully fixed in the vertical direction in the view of FIG. 1 to ensure that about half the particles in the incident beam strike stripper foil, so that proton beams 140a and 140b will have approximately equal yields. Another challenge arises because of the varying diameter (and thus varying cross-sectional area) of a particle beam. FIG. 1 shows stripper foil 130a positioned to correspond to the maximum beam diameter (i.e., the beam is widest in the vertical direction of FIG. 1 at the location of stripper foil 130a), which improves efficiency, but it is difficult to ensure such a positioning of stripper foil 130a. The positioning of stripper foil 130b along the vertical and horizontal directions of FIG. 1 does not have to be as tightly controlled as the positioning of stripper foil 130a, because stripper foil 130b handles all the remaining particles. Still, the precision required regarding positioning of stripper foil 130a is difficult to implement and presents practical challenges. Beam cross-sectional variation is difficult to control and predict, in part because magnetic field variation leads to problems of isochronism. FIG. 1 represents an ideal scenario, and often the actual beam dynamics relative to the stripper foil positioning is non-ideal because of imperfections associated with control of varying electric and magnetic fields. Furthermore, with this traditional approach only two stripper foils can be used.